This invention relates, in general, to voltage references, and more particularly to correction circuits to reduce error of a voltage reference.
Voltage references provide an accurate and stable voltage over a wide temperature. It is well known that a bandgap reference is easily integrated on existing semiconductor processes and provides an accurate reference voltage that is extremely stable over temperature. The bandgap reference provides a low temperature coefficient (TC) reference voltage by adding two voltages with opposite temperature coefficients, thus canceling the temperature dependence. The resultant voltage produced by the bandgap reference is approximately the bandgap voltage of the semiconductor material. In the case of a silicon semiconductor, the bandgap reference voltage produced is approximately 1.205 volts. The temperature dependence term canceled is generally a first order term or linear term.
Smaller second and third order temperature dependent terms affect the bandgap reference output voltage, although the largest term (linear term) of the temperature dependent terms is canceled. These remaining temperature dependent terms produce an output voltage that graphically looks like an inverted parabola. The peak of the inverted parabola is a point of zero temperature dependence (zero slope) and is typically centered at a center of a temperature range in which the bandgap reference is used. For example, assume the bandgap reference is used over a temperature range of -40 degrees centigrade to 100 degrees centigrade. The zero temperature coefficient point or peak of the inverted parabola is centered at approximately 30 degrees centigrade. Temperatures above and below 30 degrees centigrade will produce an output voltage less than the approximately 1.205 volts produced at 30 degrees centigrade.
Maximum deviations in output voltage of the bandgap reference due to temperature dependencies are small. From the example described above an output voltage deviation of approximately 8 millivolts over the temperature range (-40 to 100 degrees centigrade) can be expected. This small voltage error makes further correction of the bandgap reference extremely difficult. The problem resides in generating a small voltage with an appropriate temperature coefficient centered at the center point of the bandgap reference. Most attempts at compensation start with large voltages or currents that must be reduced or translated appropriately to generate the small correction voltage. Error produced during translation is invariably on the order of the small voltage being generated and thus is not manufacturable or accurate enough to reduce the bandgap reference temperature dependence. It would be of great benefit if an error correction circuit could be produced that is simple to manufacture, inherently produces a small error correction voltage over the usable temperature range of the bandgap reference without translation, and is easily centered with the bandgap center point.